We have read with great interest the paper by Olsen  reviewing his experience with awake craniotomies using the ‘asleep–awake’ technique. The author described this ‘asleep–awake’ technique as an alternative to the ‘asleep–awake–asleep’ technique , not only for epilepsy surgery but also for brain tumour surgery in important brain areas such as the motor cortex or areas involved in language functions. Despite the wide variability in current anaesthetic techniques for these specific intracranial procedures, our protocol is similar to the anaesthetic technique described in this paper (asleep–awake technique by combined use of propofol and remifentanil) . In his review, the author correctly focused attention on the most critical phase: awakening the patient and changing from automatic ventilation to spontaneous respiration. The patient should be adequately awakened to ensure safe spontaneous respiration without airway obstruction and without any hypoventilation. Any hypoxic or hypercapnic incident should be prevented because these would have disastrous effects on brain tension, inducing the risk of brain bulging. Coughing should also be rigorously prevented because massive brain bulging results in a very difficult, if not impossible, surgical procedure. Each additional increase in cerebral volume should be maximally prevented as lightening of anaesthesia during the wake-up phase increases the metabolic rate, the cerebral blood flow and, thus, the cerebral volume .
In order to monitor this critical step, from an ‘asleep and ventilated’ to an ‘awake and spontaneously breathing’ patient, we use bispectral index monitoring (BIS) to titrate propofol and remifentanil target-controlled infusion (TCI) administration. BIS may be useful during the sedation/anaesthesia period and also to evaluate the responsiveness during awake cortical mapping . Hans et al.  suggested that BIS correlated better with the level of sedation than the calculated effect-site concentrations of propofol. Lobo and Beiras  showed a strong correlation between BIS and level of sedation, but also with the predicted effect-site concentrations of propofol and remifentanil.
In our experience, we titrate propofol TCI to BIS values between 40 and 60 , referring to an adequate depth of the hypnotic part of anaesthesia, and remifentanil TCI to haemodynamic responses . At the start of awakening, we reduce TCI administration of both propofol and remifentanil by 10% each, until BIS values higher than 60 are attained [mean wake-up time in our experience is 18 min (14–31 min)] . When BIS values exceed 60, we change automatic ventilation to manual bag ventilation, ensuring normocapnia by end-tidal CO2 monitoring. This technique allows us to ‘manually’ feel the first inspiratory effort made by the patient and allows better prevention of any coughing. The mean propofol TCI at this time is 2.1 μg ml−1 and mean remifentanil TCI is 3.5 ng ml−1. In the following step, we simultaneously reduce TCI administration of propofol and remifentanil in order to obtain BIS values higher than 80 (mean propofol TCI 1.3 μg ml−1 and remifentanil TCI 2.1 ng ml−1). Once BIS values higher than 80 are obtained, the patient is gently stimulated and the laryngeal mask airway (LMA) is removed as the patient obeys a command. During neurological testing, we titrate TCI administration of propofol and remifentanil to BIS values between 80 and 90, according to clinical response, anxiolysis and patient comfort. At the end of neurological testing, we slowly increase the TCI administration of remifentanil and propofol according to respiratory rate and PaCO2 to allow wound closure. None of our patients reported pain, anxiety or needed to be reintubated .
In conclusion, anaesthetic care is the most unique aspect of awake craniotomy. Personal experience, careful planning and attention to detail are necessary to obtain good results .
The choice of the right combination of drugs and their best method of delivery, the adequate level of anaesthesia and consciousness for each step of the procedure and the airway management are challenging tasks for anaesthesiologists . BIS is useful in awake craniotomy even if clinical observation of vital signs and specific sedation scores are adequate to manage this type of patient . The use of BIS would be a valuable contribution to the asleep–awake technique for awake craniotomy, allowing a smooth transition from asleep to awake conditions without important compromise to vital or brain functions.
1 Olsen KS. The sleep-awake technique using propofol-remifentanil anaesthesia for awake craniotomy for cerebral tumours. Eur J Anaesthesiol 2008; 25:662–669.
2 Hans P, Bonhomme V, Born JD, et al
. Target-controlled infusion of propofol and remifentanil combined with bispectral index monitoring for awake craniotomy. Anaesthesia 2000; 55:255–259.
3 Piccioni E, Fanzio M. Management of anesthesia in awake craniotomy. Minerva Anestesiol 2008; 74:393–408.
4 Lobo F, Beiras A. Propofol and remifentanil effect-site concentrations estimated by pharmacokinetic simulation and bispectral index monitoring during craniotomy with intraoperative awaking for brain tumor resection. J Neurosurg Anesthesiol 2007; 19:183–189.
5 Mwani G, De Deyne C, Van Gompel C, et al
. Use of bispectral index (BIS) to guide the anaesthetic management for wake-up craniotomy. Eur J Anaesthesiol 2005; 22(Suppl 34):A-319.